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Some preliminary results

First results from the combination of the AAO/UKST and Marseille H$\alpha $ surveys

D. Russeil , Q. A. Parker ,, PASA, 18 (1), in press.
Next Section: Conclusion
Title/Abstract Page: First results from the
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Contents Page: Volume 18, Number 1

Subsections

Some preliminary results

Identification of a new large structure at l=298$^{\circ }$

The galactic direction l=298$^{\circ }$ is known to point between two spiral arms, implying weaker interstellar absorption towards this region. In radio emission ten sources (assumed HII regions) are observed with positive velocity in this direction (Caswell and Haynes, 1987). Using the mean rotation curve of Brand and Blitz (1993) implies that these HII regions are located outside the solar circle at about 9.5 kpc from the Sun and hence are localised in the far part of the Carina arm.

The MHS has surveyed the region and detected seven of these sources and several emission patches at the same velocity (Russeil, 1997). These HII regions, being quite remote, exhibit small projected angular sizes (typically <4 arcminutes). The MHS is unable to provide much 2-D structural information for most of them. However with the equivalent AAO/UKST H$\alpha $ survey material the detailed morphology of these regions can be clearly seen. For example we can see (fig.1) that the known HII region G297.655-0.977 at 12h01m30.3s, -63$^{\circ }$05'04" (Caswell & Haynes, 1987) and a similar but newly discovered H$\alpha $/HII region at 11h59m45s, -62$^{\circ }$22'07" exhibit sharp, bright edges from where a more diffuse emission extends. This suggests they are both located on the edge of a parental molecular cloud located to their east. Such detail is completely lost in the equivalent MHS images. It is the first time that the precise morphology of these HII regions has been seen. In addition, the associated AAO/UKST survey image (field HA135h, exposure HA17941) reveals a large, faint, diffuse emission region (fig.2) which traces a semi-circular structure of about 2$^{\circ }$ diameter centered at 12h09m24s, -62$^{\circ }$35' (298.5-0.3). This coincides in its South-West limb with the HII regions G297.506-0.765 (12h00m33.5s, -62$^{\circ }$35'), G297.655-0.977 (12h01m30.3s, -63$^{\circ }$05'04") Caswell and Haynes (1987), the region at 11h59m45s, -62$^{\circ }$22'07" and several emission patches detected at the same positive velocity by the MHS. The MHS data yields a velocity of 28 á kmá $\rm s^{-1}$ for G297.655-0.977 (fig.3) and 26 á kmá $\rm s^{-1}$ for G297.506-0.765 as well as a mean velocity of 28 á kmá $\rm s^{-1}$ for several H$\alpha $ patches which are in good agreement with the radio data (Caswell and Haynes, 1987).

These combined observations strongly suggest a link between these bright regions and the large semi-circular structure with the giant molecular cloud n$^{\circ }$26 (center 298.8-0.2, Reff=124 pc with d=9.5 kpc, Vlsr=25 á kmá $\rm s^{-1}$) listed by Grabelsky et al. (1988). Moreover the radius of the large structure (R$\sim$166 pc with d=9.5 kpc) is close that of this molecular cloud n$^{\circ }$26. Planned follow-up multi-spectral observations using the MHS instrument in other sections of this large circular structure would allow a more precise determination of this intriguing feature's nature and kinematics.

Figure: H$\alpha $ images from the AAO/UKST survey of G297.655-0.977 (12h01m30.3s,-63$^{\circ }$05'04", B1950) (left) and the HII region at 11h59m45s, -62$^{\circ }$22'07", B1950 (right). The field of view of each image is $6'\times4.6'$ with north up and east to the left. The AAO/UKST image has been obtained from a simple flatbed scan of a polaroid taken of these features from the original film (field HA135h, exposure HA17941). The detailed morphology of these regions suggests they are at the edge of the same parental molecular cloud.
\begin{figure} \begin{center} \centerline{\epsfxsize=8.5truecm\epsfbox{fig1a.ps}\epsfysize=6.45truecm\epsfbox{fig1b.ps}}\end{center}\end{figure}

Figure 2: a) H$\alpha $ image of the large semi-circular structure centered at 12h09m24s, -62$^{\circ }$35' obtained from AAO/UKST survey field HA135h (exposure HA17941). The image was provided by direct photographic reproduction from the original film at the ROE photolabs. The field of view is about

$3^\circ \times 3.3^\circ $ with north up and east to the left.
\begin{figure*} \begin{center} \centerline{\epsfxsize=18truecm\epsfbox{fig2a.ps}}\end{center}\end{figure*}

Figure 2: b) Same as figure 2a but at normal contrast. The large structure is highlighted by the long dashed lines. It seems to border the giant molecular cloud n$^{\circ }$26 of Grabelsky et al.(1988). The framed area corresponds to the MHS covered region. The small framed HII regions are (sorted by decreasing declination): the region at 11h59m45s, -62$^{\circ }$22'07", G298.22-0.331, G297.506-0.765, G298.187-0.782, G297.655-0.977. The areas within the quasi-parallel lines are zones where positive MHS velocity components were found. These zones, as well as the framed HII regions, are probably associated with the molecular cloud. Note that the other H$\alpha $ reinforcements covered by the MHS have negative velocity. The prominent HII region on the left hand side of the image is RCW64 (Vlsr=-40 á kmá $\rm s^{-1}$)
\begin{figure*} \addtocounter{figure}{-1}\begin{center} \centerline{\epsfxsize=18truecm\epsfbox{fig2b.ps}}\end{center}\end{figure*}

Figure 3: H$\alpha $ profiles from the MHS extracted for the area covered by G297.655-0.977 (left) and a nearby area (right). The components 0 and 1 are the night-sky lines while 2 and 3 are diffuse components at Vlsr = 5 á kmá $\rm s^{-1}$ and -18 á kmá $\rm s^{-1}$ met along the line of sight. Component 4 which appears only in the left hand side profile corresponds to the HII region. Its Vlsr is 28á kmá $\rm s^{-1}$.
\begin{figure} \begin{center} \centerline{\epsfxsize=8.5truecm\epsfbox{fig3a.ps}\epsfxsize=8.5truecm\epsfysize=6.5truecm\epsfbox{fig3b.ps}}\end{center}\end{figure}

Morphology and nature of extended emission features

From MHS observations, diffuse emissions often appear as uniform layers across the field or as discrete patches of H$\alpha $ emission at different velocities. The high resolution of the AAO/UKST survey allows clarification of the nature of these patches as either filaments, sharp edges of bright clouds or as more diffuse structures. Such structural information can help us to understand the different velocity components and intensity variations observed in these sources with the MHS. One good example is again given by objects detected in the AAO/UKST field HA135h (exposure HA17941) and the associated MHS H$\alpha $ emission observed around 11h50 -62$^{\circ }$15'. The MHS data in this region reveals strong localised diffuse H$\alpha $ emission extending across the entire 40 arcminute field of view (see fig.1a in Russeil, 1997) and with a mean velocity Vlsr=-34 á kmá $\rm s^{-1}$. The drastic high intensity of this H$\alpha $ emission is difficult to link with other fainter and smoother detected diffuse emissions at negative velocity seen in the vicinity. Such emission can however be attributed to the large SNR G296.05-0.5 (Caswell and Barnes, 1983). Many thin filaments around 11h49 -62$^{\circ }$20' evident in the equivalent AAO/UKST H$\alpha $ survey film (fig.4) can clearly be linked to SNR G296.1-0.7 (Longmore et al., 1977; Whiteoak & Green, 1996) which is a portion of SNR G296.05-0.5.

Figure: H$\alpha $ images of the SNR G296.1-0.7 from the AAO/UKST survey (left) and MHS (right). The AAO/UKST image has been obtained in same way as fig.2. The MHS image is from the combination of all $\lambda $-maps over the entire free spectral range of the Fabry-Perot interferometer (this corresponds to a monochromatic image of 10ö… bandwidth). The field of view of each image is about

$12.7'\times13.5'$ with north up and east to the left. The arc near the brightest star on the MHS image is an artefact. The fine detail in the AAO/UKST SNR image clarifies the nature of the H$\alpha $ emission seen on the MHS image
\begin{figure} \epsfxsize=8.5truecm\epsfbox{fig4a.ps}\epsfysize=9.0truecm\epsfbox{fig4b.ps}\end{figure}

Figure: H$\alpha $ images centred on 10h17m25s, -57o45'(B1950) between RCW48 and RCW49 from the AAO/UKST survey (field HA222h, exposure HA17974, left) and the MHS (right). The images have been obtained in same way as fig.4. The field of view of each image is about

$11.8'\times15.9'$ with north up and east to the left. The ring in the left hand quadrant of MHS image is another artefact. The AAO/UKST image clearly reveals the fine structure of the RCW48 filaments relative to the smoother patchy background. Such features are beyond the MHS instrumental resolution and so are folded in to the general patchy emission.

\begin{figure*} \begin{center} \centerline{\epsfxsize=8.5truecm\epsfbox{fig5a.ps}\epsfysize=11.1truecm\epsfbox{fig5b.ps}}\end{center}\end{figure*}

The detailed morphological study of H$\alpha $ emission permitted by the high resolution AAO/UKST survey images can also help in understanding the physical connection of different emission features when kinematic information is not alone sufficient. Consider the case of RCW48 (Rodgers et al., 1960). This region is located towards the galactic longitude 283$^{\circ }$ which is known to be tangential to a spiral arm. Hence, in this special circumstance, features can have a large range in distance without exhibiting a corresponding difference in velocity. RCW48 is a large ionized region appearing as a collection of thin filaments and exhibiting a center-edge velocity gradient (Deharveng and Maucherat, 1974; Chu, 1982). The MHS also detected this velocity gradient with the outer part having a mean velocity of about -8.6 á kmá $\rm s^{-1}$. There is also more extended patchy diffuse emission evident in the MHS data which exhibits a similar mean velocity of -6 á kmá $\rm s^{-1}$. Are these structures linked ? On the AAO/UKST film we can precisely delineate the structures belonging to RCW48 and clearly see a superposition with the more patchy background emission, with no obvious change of morphology or interaction evident between the two (fig.5). This strongly suggests that this general patchy emission and that from RCW48 are not linked. We can conclude that they are at two different distances tracing two different star-forming complexes.

Next Section: Conclusion
Title/Abstract Page: First results from the
Previous Section: The data: the AAO/UKST
Contents Page: Volume 18, Number 1

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